1. Field of the Invention
The present invention relates to an optical transmission deflector in which light is deflected by a prism which is mechanically driven via electromagnetic operation, and in particular it relates to a position sensor for detecting the position thereof. The present invention also relates to an optical scanner and a correction apparatus for correcting the scanning position in the sub-scanning direction.
This application is related to the commonly assigned application filed on Jun. 16 1998, entitled "TRANSMISSION TYPE OPTICAL DEFLECTOR", the disclosure of which is expressly incorporated by reference herein.
2. Description of the Related Art
In a known optical deflector in which an optical element is mechanically driven co deflect light, a mirror which is adapted to deflect light is reciprocally moved or the mirror is rotated. A galvano mirror is one of the typical first type in which the mirror is reciprocally moved. A polygonal mirror is another way of providing an optical deflector in which the mirror is rotated. In theory, a transmission deflector can be realized, for example, by employment of a wedge-shaped prism in place of the galvano mirror. However, in practice, since the wedge-shaped prism is heavier than the planar mirror and the position of the center of gravity of the prism varies depending on the shape thereof, oscillation tends to occur due to a weight imbalance, thus leading to difficulty in realizing a precise and high-speed optical deflector.
To this end, the assignee of the present application has proposed a precise and fast-driven optical transmission deflector using a prism (Japanese Patent Application No. 9-159932). In the proposed optical transmission deflector, the thick portion of the transmission prism is partly cut away so that the position of the center of gravity of the transmission prism is located on the axis about which the transmission prism swings.
In conventional optical scanners which have been widely used in a laser printer, etc., the laser beam emitted from a laser source and modulated in accordance with drawing data (printing data) is made incident upon the polygonal mirror (optical deflector) and is moved in the main scanning direction. The scanning laser beam is made incident upon a photosensitive body which is moved in the sub-scanning direction perpendicular to the main scanning direction through an f.theta. lens. The photosensitive body is made of, for example, a photosensitive drum or a photosensitive sheet.
In conventional optical scanners, positional deviation of the laser beam on the photosensitive body in the sub-scanning direction tends to occur due to an irregularity in the movement of the photosensitive body or a slim inclination of the reflection surfaces of the polygonal mirror, etc., thus leading to a deteriorated printing quality. The correction of the scanning position in the sub-scanning direction is carried out by the reflection type optical deflector, such as a galvano mirror. However, in the related art in which the reflection type correction system has been only employed, freedom of design is restricted and the size of the optical scanner is increased.